In the para-xylene and meta-xylene simulated bed separation process, and other simulated moving bed separation processes, bedlines are used to carry both extracted and unextracted streams. This means that these bedlines need to be flushed clean of unextracted components before the extracted streams can be withdrawn. If this is not done, the purity of the product will be impacted. In the current practice, the bedline which has been used to carry feed is flushed twice. The first time, after the feed has progressed two bedlines down, the residual feed material is flushed into the chambers and displaced with extract material from the opposite end of the adsorption cycle. The second flush pushes the extract material that is now in the bedline, along with any residual contaminants that weren't flushed initially, to the bed directly below where extract is being withdrawn.
There are several problems with the current practice. First, it couples the requirement of line flush out with the initial line flush in, though these two may not have the same optimal settings (i.e. line flush out may be optimal at approximately 100%, whereas line flush in may be optimal at >100%). Second, it leaves a bedline full of desorbent that will be the first bedline volume worth of material sent to the extract column, which is inefficient for separation, because heat is used to separate this material, though it has already been separated. Third, any feed that remained in the feed bedline is added to the chambers two full beds after the feed goes in, 2-3 minutes after the feed was introduced, at a point where some separation has already occurred, which introduces an inefficiency in feed separation. Finally, any residual contaminants from the feed that remain in the bedline after the first flush are sent to the bed directly below the extract, so there is only one bed of separation to remove these contaminants from the extract.